In prior art ground proximity warning systems such as the systems disclosed in U.S. Pat. Nos. 3,946,358 and 3,947,810, warnings are generated during a take-off or a missed approach phase of operation if the aircraft should descend at a predetermined barometric rate or lose a predetermined amount of barometric altitude prior to reaching a predetermined altitude such as 700 feet. The object of the warning is to prevent the flight crew from flying the aircraft back into the terrain directly after take-off or during a missed approach maneuver. Such accidents have occurred during periods of reduced visibility when the flight crew is unable to determine from outside visual references that the aircraft is descending. Ground proximity warning systems in use today will typically generate a voice warning in this type of situation with the voice warning consisting of the words "whoop-whoop", "pull-up" or "don't sink". This type of voice warning is normally adequate to provide a pilot with sufficient warning that the aircraft is inadvertently descending after take-off although the warning "don't sink" is considered to be better than "pull-up" since it provides the pilot with more specific information as to the nature of the hazzard faced by the aircraft. However there are situations in which it may be highly desirable to provide the pilot with more specific information as to the aircraft's actual condition. For example, during a take-off phase of flight if an engine should fail the aircraft may not be able to climb and may actually lose altitude when it is in certain flight configurations. There are certain aircraft types that under certain conditions such as an engine out configured with the landing gear down or with landing flaps down are unable to gain altitude and in fact will tend to lose altitude. Under these circumstances a simple warning of "pull-up" or "don't sink" may not provide the pilot with any useful information since he may understand quite well that he is descending but what he may not realize is that the reason he is descending is that the aircraft is not in the proper configuration for an engine out condition. It should be noted that in addition to the landing gear and flaps, flight configuration of an aircraft could include a number of other elements such as leading edge slat extensions and trim considerations. There have been incidents where the flight crew upon losing an engine during take-off have not realized that the landing gear was in an extended condition due to the large amount of work in the cockpit connected with a return to the airport under emergency conditions.
Prior art negative climb after take-off warning systems that utilize an actual loss of barometric altitude after take-off to generate a warning such as disclosed in U.S. Pat. No. 3,947,810 use an integrator to integrate barometric descent rate to get the measure of altitude loss. The integration of a barometric rate signal can lead to errors based on integrator drift or the nature of the barometric signal used.